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On Thu, 15 Dec 1994, Steve Millard wrote: > The costs of scrubber, disinfectant & other running expenses...oxygen, > helium etc I couldn't guess right now...but I'll have a better idea after the > Cis-Lunar course in early January. This depends on a number of things: Which brands? What type of scrubber material used? Whether the items are purchased retail or wholesale (since so few people will own rebreathers in the next few years, I suspect most will have access to these items at wholsale prices, one way or another.) Etc. Here are some of my calulations: Sofnolime: about US$4.50 per hour of rebreather use (moderate to heavy exertion) Lithium hydroxide: about US$5.00 per hour (same) Lithium batteries: about US$0.75 per hour Oxygen sensors: about US$1.00 per hour (depending on life expectancy) Oxygen: about US$0.15 per hour (average exertion) Helium: depends on depth of dive and how well you prevent exhaling bubbles thru the mask, but generally less than a buck an hour (independent of time, really - mostly dependant on depth, ups & downs, and gas leakage) Disinfectant: trivial. So, at current prices, you're looking at something around US$7-US$8 per hour. I'm not an authority on these figures, so some of them may be way off. (If so, I hope someone corrects me.) Also, I don't know the cost/hr for HP Sodasorb, Barium Hydroxide, or other scrubber materials. > As an aside, I got two quite different opinions regarding the mix to be > used in a fully closed-circuit rebreather during disscusions @ the Diving > Officers Conference earlier this month. When talking to Oceanic about their > 'Phibian' rebreather I asked about the conversion from nitrox to other mixed > gasses & without really thinking spoke of using trimix as the diluent. Oceanic > quickly replied that the diluent was going to be a "breathable heliox" & that > there was no point in putting nitrogen into the diluent ...as you were not > trying to save costs on a small volume of gas that was recycled. A quite > different view was put by Rob Palmer & Dorset Diving. They said that you > *would* want to use Trimix as the diluent because this would result in > *significantly* shorter decompression times. > > As I'm not yet diving on either gas mix, I couldn't comment sensibly. > Has anyone else got a comment ? How do deco times work out using open circuit > trimix or heliox (never mind the cost...) for a comparable dive ? If trimix > does work out shorter deco times, would the same conclusions be true for a > constant ppO2 closed-circuit dive ? (What I'm really asking is for someone with > a copy of 'Pro-plan', 'Abyss', 'Decom', 'Dr X' etc to run a few trial dives & > see what answers it gives. The extent of decompression advantage of trimix over heliox in a rebreather depends on a variety of factors (depth, bottom time, trimix ratio, etc.> The general rule of thumb is that longer, deeper dives are better done on heliox, and shorter, "shallower" dives are better done on trimix (from a decompression perspective). Bill Stone has an excellent article on this topic in an earlier AquaCorps (MIX, I think). The reason, of course, has to do with the fact that Helium is a "faster" molecule. On shorter dives, it gets in the tissues much faster than nitrogen, so decompression times are increased. However, on longer dives, as slower & slower tissues begin to approach saturation, heliox starts to require less decompression than trimix. That's because helium is eliminated from the tissues faster than nitrogen...so in cases of saturated tissues, decompression time is LESS for heliox than for trimix. It may seem contradictory, but when you work out the math, it makes sense. As for real times, I'm not aware of any commercially available software that can compute constant PO2 (i.e., rebreather) trimix decompression. However, I will give you some rough examples. We'll use as an example a 100msw dive for 30 minutes, with a constant PO2 of 1.4 ATM (rebreather). If heliox is breathed for the ENTIRE dive, the total dive time would be about 5.5 hours (i.e., about 5 hours decompression) (Bear in mind, these are deliberately rough, rounded-off values). If Nitrogen were used as a diluent the entire dive, the total dive time would be about 3.75 hours (3.25 hrs decompression). Breathing trimix at different ratios would yield values in-between. Of course, you probably wouldn't want to have too high a nitrogen component, because of narcosis problems. The story gets more complicated when you SWITCH diluents on decompression. On the same dive, if you used heliox for the deep portion of the dive, then switched to nitrogen as a diluent at the 40msw decompression stop, the total dive time would be about 4 hours (3.5 hrs decompression). This isn't too much more (15 min.) than the nitrogen-only dive. I don't know what the decompression would be like for a dive with trimix as a bottom diluent and nitrogen as a decompression diluent, but I don't think there would be much of an advantage...probably decompression times would increase with most practical trimix ratios (I won't go into why...basically, it's because you lose some of the advantage of switching to pure nitrogen if you have some nitrogen in the bottom-mix). The moral seems to be that using different diluents is *series* has advantages over using different diluents in *parallel* on rebreather dives. But WAIT! It gets even MORE complicated... On a rebreather, you have to start worying about diluent composition drift when using multiple diluent gases (either in parallel or in series). If your diluent is trimix, then over the course of the deep part of the dive the inspired He:N2 ratio will slowly drift in favor of N2, because the He is being removed from the loop (absorbed by your tissues) faster than the N2. The drift direction reverses at some point during decompression. The problem is also evident on a heliox-for-deep, nitrox-for-decompression rebreather dive. On decompression from such dives, the helium coming out of the tissues can accumulate in the loop. Because decompression algorithms depend on inert-gas *gradients* across alveolar membranes, decompression efficiency can be affected by these diluent drifts in a rebreather. The extent/severity of the problem ultimately depends on the ratio of the total gas volume absorbed in the body to the total volume of the loop. The problem can be minimized by periodic purging/flushing of the loop (which, of course, uses up precious diluent gas). The thing to always remember is: decompression is a very dynamic process, even when just looking at numbers on paper...it gets much MORE chaotic when you consider the various sources of variation between and within individuals., not to mention that the algorithms are just theoretical models and don't necessarily reflect what's really happening physiologically. Nope, not an exact science at all! (now..imagine the ambiguities involved with treating imprecise symptoms resulting from these chaotic processes!) (By the way, I trust NOBODY out there is stupid enough to attempt such dives using the above information as their only guide to decompression!!!) I think I've said enough (perhaps too much...) Aloha, Rich > Anyone got any spare time whilst the rubbish tv progammes are on 'the > box' over Xmas ?? What...you mean you don't like "Santa Clause vs. The Martians"? Come on...it's a classic! ;-) deepreef@bi*.bi*.ha*.or*
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